CI: Add check against PureOS upload
This lets incoming MRs be rejected/corrected with respect to PureOS builder, before letting them into the official deb repository, where corrections involve another MR and release at minimum.
Draft because it's not actually been tried out yet. Pilot deployment to be made at Librem5/debs/squeekboard!23 (merged)
Edited by Dorota Czaplejewicz